feat: support Instant string conversion

[56wj]

Purpose of the pull request

Related: #1017

Add JDK 8-compatible support for lossless conversion between java.time.Instant and UTC ISO-8601 Excel string cells.

What's changed?

• Add InstantStringConverter using OffsetDateTime for UTC-only reads and Instant.toString() for canonical writes.
• Register the converter for default writes, explicit string writes, and string reads.
• Reject non-UTC offsets to keep read/write string semantics symmetric.
• Add unit coverage for converter keys, UTC parsing, non-UTC rejection, nanosecond round trips, invalid input, and loader registration.
• Keep the implementation string-only because Excel date/number cells do not preserve timezone/offset semantics.

Verification

• mvn clean package -B -Dmaven.test.skip=false -pl fesod-common,fesod-shaded,fesod-sheet
• mvn -pl fesod-sheet -DskipTests spotless:check

Results after syncing current main on JDK 21: 924 tests, 0 failures, 0 errors, 0 skipped. The generated Surefire reports include 7 passing InstantStringConverterTest cases and 5 passing DefaultConverterLoaderTest cases.

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

[56wj]

CI on JDK 8/11 exposed a runtime compatibility difference: Instant.parse(...) rejected the +08:00 test input there, although newer JDKs accepted it. I updated the read path to OffsetDateTime.parse(...).toInstant(), which preserves the intended ISO-8601 offset normalization and remains JDK 8 compatible. The focused converter/loader tests and Spotless check pass locally on the updated commit 87ce5b2.

[bengbengbalabalabeng]

LGTM.